How Do Motorcycle Carburetors Work – NOTICE: Are your repairs covered? Compare prices to find the best deal for the motorcycle insurance you need.
Cleaning carbs is a bit tedious and tuning them gets more complicated the more carbs you have and the farther you get from a complete setup.
How Do Motorcycle Carburetors Work
If you have a motorcycle without a fuel injection system, motorcycle carburetors are responsible for delivering the combustible fuel/air mixture to the combustion chamber.
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The carbs mix and feed air and fuel through a series of different chambers, jets and ports – all of which hit different throttle positions.
The trick with motorcycle carburetors is to make sure they are properly adjusted to deliver the right mixture of fuel and air to the engine at the right time.
We will discuss them all below. You can learn how to tune your carbs on other pages throughout the motorcycle carbs section.
Mechanical motorcycle carbs feature a throttle slide that controls the height of the jet needle. As you open and close the throttle, it directly changes the position of the jet needle and the air entering the various jets.
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In a CV carb, the throttle controls a valve that changes the vacuum pressure inside the carb chamber, which is controlled by a diaphragm.
In theory, carbohydrates are very simple. They mix the gas and air in the right proportions and deliver it to the cylinders.
Motorcycle carburetors work through the venturi effect. Air enters the engine through the large hole behind the carb and creates a low pressure or vacuum system as the air flow increases.
The vacuum moves around the carb and the gas is drawn through the various jets and vaporized as it is released into the combustion chamber.
Cleaning A Carburetor In 8 Easy Steps!
Different jets and air passages in the carb are controlled by different throttle positions. At certain throttle positions, some of the systems overlap.
Each part and system of your motorcycle’s carburetor has a specific responsibility. After a while, you will be able to identify your carb symptoms and know where to look first to fix the problem.
Carb floats are made of material that floats on gas. Floats are usually made of plastic, cork or hollow brass.
Hollow floats can pinhole and fail – these can often be replaced with plastic floats from later models.
How Do Carburettors Work?
The floats are attached to a tab that presses against the float needle, which rests on the needle seat. The tab is usually flexible so that the fuel level can be adjusted up or down.
As the gas level changes, the tab attached to the floats can lift or push the needle into its seat and either stop or allow fuel to flow through the float bowls.
If the fuel level is too high, fuel can leak from the carbs or your crankcases.
Incorrect fuel levels can also cause rich or lean running conditions. Too little fuel can cause a lean mixture. Too high a fuel level can cause a rich mixture.
How Does A Carburetor Work?
The needle seat and float valve control the fuel entering the float bowl. The needle seat is usually bolted to the carb body with a thin washer and the float needle rests on the needle seat between the carb float lugs.
The float needle can wear out over time and particles can get caught between it and the seat. Both situations can cause fuel to overflow from the carb. If there is any particulate in the seat, you can sometimes correct an overflow by gently tapping the carburetor with a plastic screwdriver handle.
The idle system is responsible for approximately 0 to 25% throttle positions. The idle circuit consists of an idle jet, a screw that adjusts the air mixture, and a screw that adjusts the idle position.
The channels in the carb run from the idle jets and air screws and exit just in front of the throttle plate. As the vacuum increases, control shifts to the needle jet.
Clean & Rebuild Your Motorcycle Carburetors
The throttle slide usually has a flat side and an angled side. The throttle sliders control the fuel/air mixture from idle to about 35% open throttle in conjunction with the idle circuit.
Most throttle slides or throttle valves have a flat edge and a cut/angled edge. The cut side faces the air cleaner inlet and affects the fuel mixture in the carb.
The needle jet is located in the center of the carb above the main jet, controlling approximately 15 to 60% throttle opening. The ejection needle enters the top of the needle bundle and the main bundle is usually screwed underneath.
The firing needle is located in the center of the throttle slide. Different carbs have needles with different tapers.
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Some have grooves near the top that will allow you to change the position of the clip. Lifting the clip up effectively lowers the needle and drops the mixture from about 20 to 80% open throttle. Moving the clip down will raise where the needle sits and make the fuel/air mixture richer.
If you have a jet needle with only one clip position, you can replace it with a jet needle with multiple positions to allow customization.
The main jet is one of the largest jets on the carb and controls the fuel mixture from about 60% to full throttle.
The main jets are screwed under the needle jet and check from about 65% to full throttle.
How Do Fuel Injector And Carburetor Cleaners Work
When your motorcycle engine is cold, it cannot vaporize the fuel properly. To compensate for this, carburetors are built with systems that work to enrich the mixture at startup, effectively adding more gas so it can vaporize and ignite in the combustion chamber.
The guillotine or plate type choke mechanism is a common application on many old motorcycle carbs. Other carbs include enrichment circuits that work with float piston levers.
As the fuel passes through the jets, it mixes with the air and becomes a more misty or gas-filled fuel mixture. A carburetor is a very sensitive precision instrument designed to combine fuel and air in the correct ratio throughout the relatively dynamic operating range of an internal combustion engine.
They are also, believe it or not, very easy to understand. Although I wouldn’t say that carburetors and their tuning (matching the carburetor to a specific engine and even a specific usage scenario) are easy, the principle of their operation is quite simple and servicing is usually easy to do if the carburetor design is feasible and there is adequate access to it. Carburetors are pretty cool because we still live in an age where they are used (and arguably the most complex and best carburetor designs are all that remain in the game), but due to emissions restrictions , they are no longer developed. It is a kind of living fossil in that respect.
Motorcycle Carburetors: How They Work & What They Do
To better explain carburetor design and refinement, I’m going to do what I usually do: take you back in time to understand the simplest form of the topic we’re discussing, and then move on to all the big important milestones. I will also put pepper on some fruit to keep it from drying out.
Here is the basic idea behind a venturi. If you get this, you have a little too many carbs. display.
Like many parts of a motorcycle, the air-fuel mixing device was born from research completed in another century. In the 1730s, Daniel Bernoulli, a Swiss mathematician and physicist, discovered that air pressure decreases as speed increases. It just so happens that a nice, consistent way to force this situation to happen is to move air through a confined section of pipe. the air speeds up and the pressure drops. It was discovered in 1797 by an Italian physicist named Giovanni Venturi. He designed a tube with a smaller entrance to this restriction, in this region of low pressure. This inlet allows the tube to draw fluid into the flowing air.
That’s the whole point in a nutshell. That’s what carb does. It is a pipe that moves air with very specially placed slots that allow a very specific amount of fuel into an engine. And ideally, it also emulsifies the fuel with air – spray. (It is important to note that liquid fuel is more likely to ignite than fuel vapors suspended in air.)
Problems With Old Carburetors
This is a removable venturi from a Langsenkamp-Linkert carburetor found on many vintage Harley-Davidsons. See the area where the diameter decreases? Photo by Lemmy.
So when you “give it gas”, you’re not actually doing anything to the fuel. There is no direct connection between your right hand and the gas. What you do really comes out
. Get more air into your engine — coincidentally, due to the Venturi effect, a greater difference in air pressure allows it to carry more fuel.
If you don’t make it any further in this article, you understand what a carb does and how it does it. But as with all mechanical parts in moto, there have been developments and improvements that are very interesting. Also history and development
A Guide To Motorcycle Carburetor Maintenance
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